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Parasitic capacitance of stripboard

Discussion in 'General Electronics Discussion' started by ianb1469, Feb 22, 2010.

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  1. ianb1469

    ianb1469

    6
    0
    Feb 22, 2010
    We are always told that stripboard has a relatively "high parasitic capacitance" between tracks, but how high?

    I searched the internet expecting to find many references to "typical capacitance ... per mm of adjacent track", but not even the manufacturers seem to provide this information.

    If I knew a little about the capacitances on a protoboard, then I'd be able to know about whether I need to consider it for the sort of frequencies in the various parts of a circuit.. when to spend extra effort laying out the tracks and getting impedances worked out carefully and when to just get on with it.

    Thanks for any advice.
    Ian
     
  2. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    Good points, we can't leave that uninvestigated!
    Measured on a 307mm length of VERO 01 0042 WK 29 97:
    7.5pF between every other track.
    13pF between adjacent tracks.
    22pF between a track and two grounded tracks, one on either side.
     
  3. ianb1469

    ianb1469

    6
    0
    Feb 22, 2010
    Hi

    Many thanks for making those measurements! (Out of interest: how? Certainly my old/faithful multimeter won't measure that low).

    Anyway, based on your measurements (the 22pF one) we might estimate the capacitance to be about 0.2pF per 0.1" hole.

    It's shockingly high; you don't have to get to particularly high frequencies before the signal vanishes.

    Many thanks for the measurements, you've saved me from prototyping an idea I had on stripboard; it would simply not work :)

    Regards
    Ian
     
  4. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    I have a Philips multi-frequency RCL/ESR meter at work. I took those measurements at 1kHz. I meant to do it all week but was kept so busy I forgot until today.
    Glad to be of help. :) If you want to know the track inductance + resistance (up to 100kHz, including skin effect) i can measure that too. ;)
    What frequency would you estimate is the highest practical for these Vero boards btw.?
     
  5. ianb1469

    ianb1469

    6
    0
    Feb 22, 2010
    >What frequency would you estimate is the highest practical for these Vero boards btw.?

    Well, of course it depends...

    For my particular application, I am stuck with basically an RC filter arrangement and needing to feed signals at about 20kR impedance. If I can apply anything I once was supposed to have learnt, then at even 4 holes, the corner frequency is 10MHz. At a more realistic 20*0.1" length of track, we're down to 2MHz.

    I suppose that if you can get the load resistance down to 1k then it would be practical at 40Mhz.

    Does that sound about right? It's many years since I went to university!
     
  6. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    Yes of course, silly question, with a fixed capacitance the impedance is the all-determining factor for the corner frequency. I checked your numbers and they do add up.
    So if doing 75 ohm stuff then 20 holes could possibly be good up to around 500MHz.
     
  7. ianb1469

    ianb1469

    6
    0
    Feb 22, 2010
    500MHz - I suspect that's rather optimistic :)
     
  8. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    Seeing how UHF circuits are made up I guess you are right.. :)
     
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